it is deliberately and of set purpose that I extend my hand
and perceive it; what happens in sleep does not appear so
clear nor so distinct as does all this.  But in thinking over
this I remind myself that on many occasions I have in sleep
been deceived by similar illusions, and in dwelling carefully
on this reflection I see so manifestly that there are no
certain indications by which we may clearly distinguish
wakefulness from sleep that I am lost in astonishment.  And my
astonishment is such that it is almost capable of persuading
me that I now dream.

     Now let us assume that we are asleep and that all these
particulars, e.g. that we open our eyes, shake our head,
extend our hands, and so on, are but false delusions; and let
us reflect that possibly neither our hands nor our whole body
are such as they appear to us to be.  At the same time we must
at least confess that the things which are represented to us
in sleep are like painted representations which can only have
been formed as the counterparts of something real and true,
and that in this way those general things at least, i.e. eyes,
a head, hands, and a whole body, are not imaginary things, but
things really existent.  For, as a matter of fact, painters,
even when they study with the greatest skill to represent
sirens and satyrs by forms the most strange and extraordinary,
cannot give them natures which are entirely new, but merely
make a certain medley of the members of different animals; or
if their imagination is extravagant enough to invent something
so novel that nothing similar has ever before been seen, and
that then their work represents a thing purely fictitious and
absolutely false, it is certain all the same that the colours
of which  this is composed are necessarily real.  And for the
same reason, although these general things, to with, [a body],
eyes, a head, hands, and such like, may be imaginary, we are
bound at the same time to confess that there are at least some
other objects yet more simple and more universal, which are
real and true; and of these just in the same way as with
certain real colours, all these images of things which dwell
in our thoughts, whether true and real or false and fantastic,
are formed.

     To such a class of things pertains corporeal nature in
general, and its extension, the figure of extended things,
their quantity or magnitude and number, as also the place in
which they are, the time which measures their duration, and so
on.

     That is possibly why our reasoning is not unjust when we
conclude from this that Physics, Astronomy, Medicine and all
other sciences which have as their end the consideration of
composite things, are very dubious and uncertain; but that
Arithmetic, Geometry and other sciences of that kind which
only treat of things that are very simple and very general,
without taking great trouble to ascertain whether they are
actually existent or not, contain some measure of certainty
and an element of the indubitable.  For whether I am awake or
asleep, two and three together always form five, and the
square can never have more than four sides, and it does not
seem possible that truths so clear and apparent can be
suspected of any falsity [or uncertainty].

     Nevertheless I have long had fixed in my mind the belief
that an all-powerful God existed by whom I have been created
such as I am.  But how do I know that He has not brought it to
pass that there is no earth, no heaven, no extended body, no
magnitude, no place, and that nevertheless [I possess the
perceptions of all these things and that] they seem to me to
exist just exactly as I now see them?  And, besides, as I
sometimes imagine that others deceive themselves in the things
which they think they know best, how do I know that I am not
deceived every time that I add two and three, or count the
sides of a square, or judge of things yet simpler, if anything
simpler can be imagined?  But possibly God has not desired
that I should be thus deceived, for He is said to be supremely
good.  If, however, it is contrary to His goodness to have
made me such that I constantly deceive myself, it would also
appear to be contrary to His goodness to permit me to be
sometimes deceived, and nevertheless I cannot doubt that He
does permit this.

     There may indeed be those who would prefer to deny the
existence of a God so powerful, rather than believe that all
other things are uncertain.  But let us not oppose them for
the present, and grant that all that is here said of a God is
a fable; nevertheless in whatever way they suppose that I have
arrived at the state of being that I have reached¥whether they
attribute it to fate or to accident, or make out that it is by
a continual succession of antecedents, or by some other
method¥since to err and deceive oneself is a defect, it is
clear that the greater will be the probability of my being so
imperfect as to deceive myself ever, as is the Author to whom
they assign my origin the less powerful.  To these reasons I
have certainly nothing to reply, but at the end I feel
constrained to confess that there is nothing in all that I
formerly believed to be true, of which I cannot in some
measure doubt, and that not merely through want of thought or
through levity, but for reasons which are very powerful and
maturely considered; so that henceforth I ought not the less
carefully to refrain from giving credence to these opinions
than to that which is manifestly false, if I desire to arrive
at any certainty [in the sciences].